1. Field of the Invention
The present invention is directed to an improved process for the production of ammonia. More specifically, the present invention involves a low energy synthesis process for producing ammonia. One aspect is that the ammonia synthesis is carried out at low pressures, i.e., at pressures less than 100 atmospheres (atms.). Another aspect is that the ammonia is recovered from the synthesis effluent stream by scrubbing with water or a water absorption system and then distilling the ammonia-water mixture using the low level heat recovered from the process used for producing the synthesis gas. Still another aspect is to refrigerate the product ammonia in an ammonia absorption system and using a single distillation system to reconstitute the absorbent and purify the ammonia for both the water absorption system and the refrigeration system.
2. Prior Art
A commercial ammonia process is disclosed in U.S. Pat. No. 3,441,393. In such a process, ammonia synthesis gas is produced in a series of steps. A raw synthesis or hydrogen-rich gas is formed by partial oxidation or primary reforming of a hydrocarbon, which can be either liquid or gaseous at normal temperatures and pressures. The carbon monoxide in the raw synthesis gas is converted in a shift conversion zone to carbon dioxide and further hydrogen. The hydrogen-rich gas is then scrubbed to remove the carbon dioxide. Further purification steps such as methanation may be used to obtain a synthesis gas having hydrogen and nitrogen in approximately a 3 to 1 ratio. The synthesis gas is converted to ammonia and the ammonia is recovered by using mechanical refrigeration compressors.
The pressure in commercial ammonia synthesis loops generally exceed 100 atmospheres and most exceed 150 atmospheres. However, pressures of 1 to 68 atmospheres are disclosed in U.S. Pat. No. 2,550,389, pressures of 20 to 400 atmospheres are disclosed in U.S. Pat. No. 3,368,869 and pressures of 40- 120 atmospheres are disclosed in U.S. Pat. No. 3,957,449, although no commercial plant is known which operates at low pressures, i.e., less than 100 atms.
U.S. Pat. No. 3,397,959 discloses an isopeistic process (i.e., essentially same pressure throughout the process) and apparatus for the production of ammonia. In this process the steps of preparing the synthesis gas and the synthesis of the ammonia are all conducted at a pressure of 80 to 220 atmospheres.
A water recovery process for ammonia is disclosed in an article by Pagani and Zardi, "New separation process gives cheaper ammonia," published in Hydrocarbon Processing, July, 1972 on pages 106 to 110.
In a specific process using a closed ammonia-water absorption-refrigeration cycle, a process is disclosed in U.S. Pat. No. 3,743,699 which does not require mechanical compression of the refrigerant.